Preheat oven to 375 F. Line baking sheets with parchment paper or silicone baking mat.
Melt butter in a heavy saucepan.
Add brown sugar to a large bowl.
Pour melted butter over sugar.
Stir to combine butter and sugar. Let stand 5 minutes.
Add egg and egg yolk; stir to combine.
Stir in vanilla extract.
Add flour, cocoa powder, baking soda, and salt. Stir until just combined.
Stir in chocolate chips.
Using a medium cookie scoop (1.5 tablespoon), drop scoops of dough 2 inches apart on prepared cookie sheet.
Bake for about 8-11 minutes, or until the edges of the cookies are set.
Let cool on cookie sheet for 5 minutes. Remove to wire rack to finish cooling.
*Correctly measuring the ingredients is key. I suggest either weighing the ingredients or aerating the flour (sifting or fluffing) and then scooping and leveling. Packing in too much flour will cause the cookies not to spread or to be dry and crumbly. **Can substitute 4 ounces finely chopped baking chocolate for chocolate chips. I like to use semisweet in this recipe. If you prefer a less sweet cookie, you can use bittersweet chocolate. If you prefer a sweeter cookies, try milk chocolate. ***I suggest stirring this recipe together by hand rather than using a hand or stand mixer. ****Nutrition values are estimates.